The Session Initiation Protocol (SIP) is an application layer control protocol for creating, modifying, and terminating sessions with one or more participants. The sessions include file transfers, Internet telephone calls, multimedia distribution, and multimedia conferences. SIP may be used to locate a participant, invite a participant to engage in communications, determine media parameters to use, establish a session, and maintain a session. The session itself typically uses another protocol, referred to as a “secondary protocol,” to perform the communications. A session can be used for a variety of data transfer applications, including the transfer of a file between participants. Request for Comments (RFC) 3261, available on the Internet (e.g. at address ietf.org/rfc/rfc3261.txt) provides a detailed description of SIP.